Mr. Jones is a 78-year-old gentleman who presents to the clinic with a chief complaint of fever, chills and cough. He also reports some dyspnea. He has a history of right sided CVA, COPD, dyslipidemia, and HTN. Current medications include atorvastatin 40 mg po qhs, lisinopril, and fluticasone/salmeterol. He reports more use of his albuterol rescue inhaler. Vital signs Temp 101.8 F, pulse 108, respirations 21. PaO2 on room air 86% and on O2 4 L nasal canula 94%. CMP WNL, WBC 18.4. Physical  reveals thin, anxious gentleman with mild hemiparesis on left side due to CVA. HEENT WNL except for diminished gag reflex and uneven elevation of the uvula, CV-HR 108 RRR without murmurs, rubs, or click, no bruits. Resp-coarse rhonchi throughout lung fields. CXR reveals consolidation in right lower lobe. He was diagnosed with community acquired pneumonia (CAP).

Question: Patient was hypoxic as evidenced by the low PaO2. Explain the pathologic processes that caused this patient’s hypoxemia.